Digital electronic devices use random access memory circuits to store digital information. Modern digital electronic devices such as personal computer systems, tablet computers, and cellular telephones are increasingly using larger and larger amounts of random access memory circuits to store digital data. With larger amounts of random access memory circuits there is a greater probability of a manufacturing defect occurring within the memory section of a complex integrated circuit that may require the entire integrated circuit to be discarded. Furthermore, a subtle manufacturing defect within the memory section of an integrated circuit may not be accurately detected immediately after the integrated circuit has been manufactured thus causing significant reliability problems for the integrated circuit at a later time.
To address manufacturing problems with digital random access memory circuits, most memory manufacturers provide integrated Built-In Self-Test (BIST) and Built-In Self Repair (BISR) systems for the memory system. Built-In Self-Test (BIST) systems are used to test random access memory circuits for memory defects. Built-In Self Repair (BISR) systems are used to reroute memory circuits away from defective memory circuit cells and instead to a set of spare memory circuit cells that are not defective.
Existing Built-In Self-Test (BIST) and Built-In Self Repair (BISR) systems have significantly improved memory reliability. However, there still are some situations wherein subtle memory defects can still cause problems. For example, some manufacturing defects may not be detectable at the time of manufacture but only begin to appear at a later time. Other defects may not occur during initial power-up test but may begin to cause problems during operation. Therefore, it would be desirable to improve memory testing systems such that defects within random access memory circuits can be identified in a more reliable manner.